[count] 1 a : a person who sleeps in a particular way
▪ a light sleeper [=someone who does not sleep well and wakes up easily] ▪ a heavy sleeper [=someone who sleeps well and does not wake up easily] ▪ a sound sleeper
b : someone who is asleep
▪ Sleepers were awakened by the sound of a loud crash. 2 a : a place or piece of furniture that can be used for sleeping — usually used before another noun ▪ the sleeper cab of a truck ▪ Our guests will use the sleeper sofa. [=a sofa that folds out to form a bed]
b : sleeping car 3 chiefly US, informal : someone or something that suddenly becomes successful in a way that was not expected
▪ He may turn out to be the sleeper in this year's election. ▪ The movie was the summer's sleeper. ▪ The movie became the sleeper hit of the summer. 4 : a spy who lives and works for a long time like an ordinary person among the people he or she is spying on — often used before another noun ▪ a sleeper agent ▪ The police discovered a sleeper cell [=a small organized group of spies] that had been operating in the city for years. 5 US : a piece of clothing that covers the entire body including usually the feet and that is worn by a baby or small child 6 Brit : one of the heavy beams to which the rails of a railway are attached : tie